High-Resolution Electron Microscopic Study of Ba7Sc6Al2O19 and Related Phases

It is shown that the crystal structures of the compound Ba7Sc6Al2O19 and of related structures which belong to the class of hexagonal perovskites can be interpreted as mixed-layer structures consisting of the intergrowth of structural blocks (or lamellae) of β-Ba2ScAlO5 and Ba3Sc4O9, two other hexagonal perovskites. The block sequences have been imaged directly by means of high-resolution electron microscopy. The characteristic geometrical features of the structural blocks allow unambiguous identification of the different sequences. The image interpretation was confirmed by computer-simulated images. The structures can alternatively be considered as interface modulated structures derived from a cubic stacking of close-packed BaO3 layers in which periodic stacking faults and twin boundaries are introduced. These interfaces are stabilized by B cations.